1. Field of the Invention
This invention relates to a driving circuit for an acoustic printer and an acoustic printer, and particularly to the driving circuit of an acoustic printer and the acoustic printer for realizing high speed operation and high image quality.
2. Description of the Related Art
As an apparatus for recording an image by making ink liquid into small grains (so-called liquid drop) and flying the same on a recording medium to form a dot, an ink jet printer has been put to practical use heretofore. A known acoustic printer uses the operation of an acoustic transducer in a device for flying the liquid drop on the recording medium.
As an example, Japanese Patent Application Laid Open No. 5-278218 corresponding to U.S. Pat. No. 5,191,354 discloses a technique. The acoustic printer using an acoustic transducer is adapted to produce periodic perturbation on the free surface of liquid ink with a suitable excitation frequency. When the amplitude of vibration pressure is equal to or higher than the critical rise amplitude level, one or more standing capillary waves are generated on the free surface of liquid ink, thereby flying a liquid drop on the recording medium. To produce such perturbation, the transducer is connected to a driver to be driven.
Further, Japanese Patent Application Laid Open No. 8-187853 corresponding to U.S. Pat. No. 5,589,864 discloses a technique where a piezoelectric device driven by an RF signal is used as a transducer. In this technique, a PIN diode or a varactor is connected in series to the piezoelectric device, and in the case of the varactor, the impedance is changed to switch on and off the RF signal, thereby controlling ink injection.
To control the RF signal, the applicant of the invention has proposed a technique (Japanese Patent Application Laid Open No. 11-72211) of generating an a.c. signal in a piezoelectric element without an a.c. signal source concerning the RF-controller and the RF driving circuit. In this technique, the inductance connected in parallel to the piezoelectric element constitutes a parallel resonance circuit, and electric charges from charge storage means and energy based on the resonance circuit are alternately supplied by switching means to discharge ink, so that it is not necessary to always supply an a.c. signal so as to reduce power consumption.
Generally, to obtain a print image of high image quality in a printer, improvement in resolution and gradient is demanded. In the acoustic printer, the resolution can be improved by making an ink drop minute, and the gradient can be improved by increasing or decreasing the diameter of an ink drop or superposing minute ink.
Concerning the improvement in resolution, there is direct proportionality between the wavelength of an ultrasonic wave propagating through an ink chamber filled with ink and the discharged ink drop, and the size of an ink drop can be varied by controlling the wavelength of an ultrasonic wave. Thus, the resolution can be improved.
Concerning the improvement in gradient, there have been proposed a technique where the frequency/amplitude/duration of the RF signal is changed to obtain ink drops different in diameter and a technique where plural ink drops discharged by starting the piezoelectric element plural times are superposed on the same pixel on a recording medium. Japanese Patent Application Laid Open No. 8-290587 discloses a technique where an ink drop group continuously discharged by one drive of the piezoelectric element is superposed On the same pixel on a recording medium.
The technique of superposition recording plural or one group of ink drops, however, has the problem that essentially the print speed is lowered.
For example, in the conventional superposition recording, the dot diameter after superposition is calculated by the following expression.
(Dot diameter of superposition)=(Unit dot diameter)xc3x97(The one third power of superposition frequency)
Accordingly, to obtain double dot diameter, the discharge time for eight times is required. Japanese Patent Application Laid Open No. 8-290587 discloses:
(Time for obtaining N-number of ink drop groups) less than Nxc3x97(Time for obtaining one ink drop),
however, at least the time 1.2 or more times as much as the time for obtaining one ink drop is required for obtaining two ink drop groups.
Then the technology for changing the size of an ink drop by RF frequency is desired to realize high speed and high image quality recording. However, in the conventional technology for changing the size of an ink drop, the driving mode of the piezoelectric element needs a high output RF amplifier. Therefore, the cost is increased and the apparatus is increased in size.
As another driving mode of the piezoelectric element, the applicant of the invention has proposed the technology disclosed in Japanese Patent Application Laid Open No. 11-72211. In the technology disclosed in Japanese Patent Application Laid Open No. 11-72211, the driving circuit can be reduced in size and the cost of the apparatus can be held down to the lowest by adopting TANK circuit where an inductance and a capacity component of a piezoelectric element constitute a parallel resonance circuit.
The TANK circuit, however, has the problem that the resonance frequency is single and the gain is remarkably lowered for a different RF frequency so that sometimes an ink drop can not be discharged.
The present invention has been proposed in view of the above circumstances and an object of the present invention is to provide a driving circuit for an acoustic printer which enables high speed and high image quality recording and reduction of size and cost.
Other object of the present invention is to provide an acoustic printer using a driving circuit.
The first aspect of the present invention is a driving circuit for an acoustic printer adapted to form an image on a recording medium by an ejector for discharging stored ink by an ultrasonic wave generated from a piezoelectric element in response to the supply of an a.c. signal. The driving circuit comprises a resonance circuit which includes an inductance connected in parallel to the piezoelectric element, has a plurality of predetermined resonance frequencies, and is set to one resonance frequency among the plural resonance frequencies; and setting means for setting the resonance frequency of the resonance circuit.
According to the first aspect of the present invention, the resonance circuit includes the inductance connected in parallel to the piezoelectric element, and a parallel resonance circuit (so-called TANK circuit) is constituted by the capacity component contained in the piezoelectric element and the inductance. In this arrangement, according to a supplied designated a.c. signal, an a.c. signal on a resonance frequency in the parallel resonance circuit is output. When the signal on the resonance frequency is input to the piezoelectric element, the ultrasonic waves are generated from the piezoelectric element and stored ink is discharged from an ejector to form an image on a recording medium.
In this aspect, the resonance circuit has plural resonance frequencies, and the plural resonance frequencies are set by setting means, whereby the resonance frequency of the resonance circuit is changed, the frequency of an a.c. signal supplied to the piezoelectric element is changed, and the frequency for driving stored ink is changed. Accordingly, the size of an ink drop discharged above the critical point of surface tension of ink is changed. In other words, the resonance frequency of the resonance circuit is set by the setting means to control the size of an ink drop discharged from an ejector.
That is, the resolution can be improved by controlling the size of the ink drop and also the gradient can be improved. Since an image is recorded by changing the frequency of an a.c. signal supplied to the piezoelectric element, an image can be recorded with good image quality without superposition of dots so as to record an image at high speed.
Furthermore, since the driving circuit of an acoustic printer adopts the tank circuit comprising a parallel resonance circuit thus formed by the inductance and the capacity component contained in the piezoelectric element, the driving circuit of the acoustic printer can be reduced in size and the cost of the apparatus can be held down.
The second aspect of the present invention is a driving circuit for an acoustic printer according to the first aspect, wherein the resonance circuit includes inductance means which can be set to one inductance among a plurality of predetermined inductances, and the setting means is adapted to set the resonance frequency by setting the inductance of the inductance means.
According to the second aspect of the present invention, the resonance circuit includes inductance means which can be set to one inductance among a plurality of predetermined inductances, and the inductance of the inductance means is set by the setting means to set one of plural resonance frequencies.
The inductance means is formed by plural inductance elements different in inductance and switching means for switching one of the plural inductance elements. The setting means may be constituted to force the switching means to switch one inductance element. The inductance is thus selectively switched to easily vary the ink drop size.
Concrete setting of inductance in the inductance means and setting means may be performed by switch means controllable from the outside. For example, plural serially connected inductance and switch means are used to be respectively connected in parallel to each other and connected in parallel to the piezoelectric element, and the respective switch means is controlled externally to change the resonance frequency. In this case, as switching means, a transistor element not grounded may be used.
The third aspect of the present invention is a driving circuit for an acoustic printer of the first or second aspect of the present invention, wherein the setting means is adapted to set the plural resonance frequencies according to image data expressing an image formed on the recording medium.
According to the third aspect of the present invention, the setting means controls setting of plural resonance frequencies according to the image data (e.g. characters, lines, photo images or the like) expressing an image formed on the recording medium to enable high speed and high image quality recording according to the image.
For example, according to the image data expressing an image formed on a recording medium, plural inductances of the inductance means described in the second aspect are set to output ink of an ink drop size (dot size) depending on the resonance frequency varying with the inductance from an ejector. That is, the dot size is varied to enable high speed and high image quality recording according to an image.
The invention described in the fourth aspect of the present invention is a driving circuit for an acoustic printer adapted to form an image on a recording medium by an ejector for discharging stored ink by ultrasonic waves generated from a piezoelectric element. The driving circuit comprises generating means for generating a positive polarity pulse signal on a designated frequency for generating an a.c. signal supplied to the piezoelectric element and a negative polarity pulse signal complementary with the positive polarity pulse signal; selecting means for selecting at lease one of the positive polarity pulse signal and the negative polarity pulse signal; and a.c. signal generating means for generating an a.c. signal to be supplied to the piezoelectric element according to the pulse signal selected by the selecting means.
According to the fourth aspect of the present invention, the generating means generates a positive polarity pulse signal with a designated frequency for generating an a.c. signal supplied to the piezoelectric element and a negative polarity pulse signal complementary with the positive polarity pulse signal. The selecting means selects at least one of the positive polarity pulse signal and the complementary negative polarity pulse signal generated by the generating means, and according to the selected pulse signal, an a.c. signal supplied to the piezoelectric element is generated by the a.c. signal generating means. Thus, at least one of the positive polarity pulse signal and the complementary negative polarity pulse signal is selected by the selecting means and an a.c. signal is generated according to the selected pulse signal, but in the case where the positive polarity pulse signal and the complementary negative polarity pulse signal are selected, the respective pulse signals are composed, and according to the composite pulse signal, an a.c. signal is generated by the a.c. signal generating means. That is, the positive polarity pulse signal and the complementary negative polarity pulse signal are composed so that the composite pulse signal is a different pulse signal different in frequency from the positive polarity pulse signal or the complementary negative polarity pulse signal, and a.c. signals with different frequencies can be generated depending on the case of selecting the positive polarity pulse signal and the complementary negative polarity pulse signal and the case of selecting one pulse signal of them. Accordingly, plural a.c. signals different in frequency can be generated by the selecting means and the a.c. signal generating means.
The positive polarity pulse signal or the complementary negative polarity pulse signal may be pulse signals which have the same period and phase difference, or pulse signals having different periods. When the period is different, the phase difference is independent directly, but it is desirable that the rising and the falling of the signal agree with each other at the starting point in the case of deciding to start.
Ultrasonic waves are generated from the piezoelectric element by supplying an a.c. signal generated by the a.c. signal generating means to the piezoelectric element and stored ink is discharge by the ejector to form an image on the recording medium. At this time, a.c. signals with different frequencies can be generated by the selecting means and the a.c. signal generating means so that the frequency of an ultrasonic wave generated from the piezoelectric element can be changed and the frequency of vibrating stored ink can be changed. That is, the size of an ink drop discharged on exceeding the critical point of surface tension of ink can be changed.
Accordingly, the size of an ink drop discharged from the ejector can be controlled by the selecting means and the a.c. signal generating means. That is, the size of an ink drop is thus controlled so that the resolution can be improved and also the gradient can be improved. Further, since an image is recorded by changing the frequency supplied to the piezoelectric element, an image can be recorded with good image quality without superposition of dots and image recording can be performed at high speed. The frequency of an a.c. signal can be selectively switched so that the size of an ink drop can be easily changed.
The selecting means selects one of the positive polarity pulse signal and the negative polarity pulse signal, whereby an a.c. signal with the main frequency can be generated. For example, between the positive polarity pulse signal and the negative polarity pulse signal complementary with the positive polarity pulse signal, one signal with the duty ratio of substantially 50% is selected to generate an a.c. signal with the main frequency among the plural signals different in frequency.
On the other hand, the selecting means selects both of the positive polarity pulse signal and the negative polarity pulse signal, whereby an a.c. signal with the sub-frequency can be generated. For example, a positive polarity pulse signal and a negative polarity pulse signal with the duty ratio of substantially 25 with the phase shift of xcfx80xe2x96xa1 are selected to generate an a.c. signal with the sub-frequency among the plural signals different in frequency. In the above examples, the sub-frequency is lower than the main frequency.
The fifth aspect of the present invention is a driving circuit according to the fourth aspect, wherein the selecting means is adapted to perform the selection according to image data expressing an image formed on a recording medium.
According to the fifth aspect of the present invention, the selecting means described in the fourth aspect selects at least one of the positive polarity pulse signal and the complementary negative polarity pulse signal according to image data (e.g. characters, lines, photo images or the like) expressing an image formed on the recording medium, whereby the frequency of an a.c. signal supplied to the piezoelectric element can be controlled to output ink of an ink drop size (dot size) according to the frequency of the a.c. signal supplied to the piezoelectric element from the ejector. That is, the dot size is varied to enable high speed and high image recording according to the image.
The sixth aspect of the present invention is an acoustic printer which is equipped with the driving circuit of an acoustic printer described in at least one of the first, second or third aspect to provide a high speed high image quality, small-sized and low-cost acoustic printer.
The seventh aspect of the present invention is an acoustic printer which is equipped with the driving circuit of an acoustic printer described in at least one of the fourth or sixth aspect to provide a high speed high image quality, small-sized and low-cost acoustic printer.